ABELtronics AMP-901 + MON-901 Amplifier Test Results

My first hi-fi amplifier completed when I was 17, and still used as my reference amp to this day.

The amplifier is a standard dual-VAS MOSFET class AB stereo design with oversized power supply and bridging function. Built in 2001, the amp is still used as my reference hi-fi amplifier to this day and continues to sound lovely.

The amplifier is essentially in two parts - the 'AMP-901' is the amplifier itself comprising amplifier PCB and dual-transformer power supply, and the 'MON-901' is what I called an amplifier monitor.

The MON-901 displays the amplifier heatsink temperature and supply rail voltages, has internal automatic temperature controlled fans for forced-air cooling of the AMP-901 (it is designed to sit on top of the amplifier) and also acts as a switched sequential power distribution block complete with RFI filtration. The temperature at which the fans begin to operate is adjustable from the front panel. The images below show four 13A sockets on the rear of the case which are sequentially activated at power-on. They are marked 'Amplifier', 'Pre-amp', 'RIAA Eq' and 'Turntable'. I thought at the time I was going to be a vinyl enthusiast, but CD became my medium of choice. Each socket features its own mains filtration and transient voltage suppression.

The front of the MON-901 has the red LED panel meter and a 'select' switch allowing the meter to display either the amplifier heatsink temperature, positive rail voltage and negative rail voltage. A further press of the select switch turns the display and all the front panel LEDs off on both units. This is good for watching films in subdued light... The circuitry is extremely complicated and could have been simplified greatly if I'd had the knowledge then that I do now.

The amplifier uses some rather fancy components and output devices, but is essentially a modified Hitachi application note MOSFET circuit. The rail voltage is about +/- 60V and the output devices are Exicon MOSFETs. The amp operates in high-biased class AB mode, and generates predominantly even order harmonic distortion giving it a valve-like sonic character.

Performance is very good, as is the performance in general with this type of circuit; the amplifier clips gracefully and shows no signs of instability even when driven with nasty stimuli. The distortion on the whole is very low, but it rises slightly at low frequencies mostly due to an undersized input coupling capacitor and low value electrolytic in the feedback arm (as evidenced by a statistically insignificant increase in 40Hz THD from 8R to 4R). Unfortunately I made the amplifier virtually impossible to disassemble and modify, so a lesson learnt. The distortion rise at high frequencies is characteristic of almost any amplifier and is due to the reduction in open loop gain at HF and the VASes having to source current into the decreasing impedance of the dominant pole compensation capacitors. Both issues can be minimised with design modifications.

Mechanically, the amplifier is screened from the power supply with aluminium sheet and there is a separate mains transformer for the control and bridging circuitry. It is mounted in a 4U 19" enclosure, and occupies 6U with the MON-901 sitting on top of it.

It's the only one in the world, and it will never be for sale.

Freq

8R Performance

4R Performance

2R Performance

Watts

THD%

Watts

THD%

Watts

THD%

40Hz

143

0.059

226

0.060

-

-

1kHz

140

0.024

220

0.034

-

-

10kHz

142

0.103

219

0.170

-

-

All results taken with sine wave input into a resistive dummy load of 8, 4 and 2 ohms (where applicable), two channels simultaneously driven (unless otherwise stated), at the threshold of clipping. These results should be considered maximum 'continuous RMS' power ratings (>5 seconds). Distortion measurements, labelled THD%, are taken with an HP8903A audio analyser (80kHz Bandwidth).

1 Power to weight ratio is calculated by taking the average of the power measurements at 4 ohms, multiplying by the number of driven amp channels, and then dividing this value by the weight of the amplifier.

Click the images below for full-size versions

Description of Images:

The MON-901 sits on top of the AMP-901.

The monitor displaying heatsink temperature...

...positive rail voltage...

...and negative rail voltage.

Closeup of the panel meter, select button, and indicator LEDs.

Amplifier rear. There is a separate phono input at the bottom left for bridged mode, handy for driving passive subwoofers.

Amplifier monitor with it's four sockets.

Another shot of the rear of both units. The vinyl sticker on the rear of the MON-901 began to fall off about 5 years ago.

Amplifier rear.

Amplifier mains plugged into the monitor, and small link cable delivers the temperature and voltage readings.